Trilobite

Trilobites are a group of formerly numerous marine animals that disappeared in the Permian–Triassic extinction event, though they were in decline prior to this killing blow, having been reduced to one order in the Late Devonian extinction.

Trilobites belong to the phylum Arthropoda. They share arthropod characteristics with other members of the phylum, including insects, arachnids, crustaceans, centipedes, and horseshoe crabs. Within the phylum, the classification of arthropods is the subject of some debate. For the purposes of this article, we will follow the classification scheme published in Borror and DeLong’s Introduction to the Study of Insects and place the trilobites in their subphylum: the Trilobita.

Description

Although several thousand species of trilobites have been identified from the fossil record, most are easily recognized as trilobites. Their bodies are somewhat ovoid and slightly convex. The trilobite body is divided longitudinally into three regions: an axial lobe in the middle and a pleural lobe on either side of the axial lobe (see figure above). Trilobites were the first arthropods to secrete exoskeletons of hardened calcite, which is why they left such a rich fossil record. Living trilobites had legs, but their legs were made of soft tissue, so they are rarely preserved in fossil form. The few complete trilobite fossils that have been found show that trilobite appendages were often biramous, carrying both a leg for locomotion and a feathery gill, presumably for respiration.

The head region of the trilobite is called the cephalon. A pair of antennae extended from the cephalon. Some trilobites were blind, but those that could see often had conspicuous, well-formed eyes. Curiously, trilobite eyes were not made of organic soft tissue, but of inorganic calcite, just like the rest of the exoskeleton. Trilobites were the first organisms with compound eyes (although some sighted species had only simple eyes). Exoskeleton during molting.

The middle part of the trilobite body, just behind the cephalon, is called the thorax. These thoracic segments were articulated, allowing some trilobites to curl up or roll up like a modern-day bug. The trilobite probably used this ability to defend itself from predators. The hind end or tail of the trilobite is known as the pygidium. Depending on the species, the pygidium may consist of a single segment or many (perhaps 30 or more). The segments of the pygidium were fused, making the tail rigid.

Diet

Because trilobites were marine animals, their diet consisted of other marine life. Pelagic trilobites could swim, although probably not very fast, and probably fed on plankton. The larger pelagic trilobites may have preyed on crustaceans or other marine organisms they encountered. Most trilobites were bottom dwellers and probably collected dead and decaying matter from the sea floor. Some benthic trilobites probably disturbed sediments to filter out edible particles. Fossil evidence shows that some trilobites plowed the seafloor in search of prey. Trace fossils of trilobites show that these hunters were able to chase and catch sea worms.

Life history

Trilobites were among the first arthropods to inhabit the planet, based on fossil specimens dating back nearly 600 million years. They lived throughout the Paleozoic Era, but were most abundant during the first 100 million years of that era (especially during the Cambrian and Ordovician periods). In just 270 million years, trilobites were gone, having gradually declined and finally disappeared at the end of the Permian period.

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